1. Field of the Invention
The present invention relates to an optical scanning device that scans an image surface within an image forming system by focusing a light beam from a light source on the image surface. Moreover, the present invention relates to an image forming system in which the optical scanning device is provided. The optical scanning device of the present invention is appropriate for use in digital copiers, laser printers and laser facsimiles.
2. Description of the Related Art
In many conventional optical scanning devices, the optical parameters of each optical scanning device are optimized in order to provide good performance for all the necessary optical characteristics of the optical scanning device. However, the optical characteristics, which must be corrected by the optical parameter optimization, include the main-scanning or sub-scanning field curvature, the wavefront aberration, the linearity and the scanning line deviation. It is difficult to achieve an easy-to-manufacture, cost-effective configuration of the optical scanning device while providing good performance for all the necessary optical characteristics. In many conventional optical scanning devices, the easy-to-manufacture, cost-effective configuration is traded off for obtaining a reasonable level of the necessary optical characteristics.
For example, even if an optical scanning device has a certain amount of linearity, it is possible to provide the cost-effective configuration and the uniform-velocity characteristics for the optical scanning device. Japanese Laid-Open Patent Application No. 10-333069 discloses such an optical scanning device. The optical scanning device of the above document provides good performance for the uniform-velocity characteristics (the linearity=±0.1%). Moreover, Japanese Laid-Open Patent Application No. 11-64760 discloses a similar optical scanning device. The optical scanning device of the above document provides good performance for the uniform-velocity characteristics (the linearity=−0.1% to +0.2%).
The optical parameter optimization is a conceivable method for increasing the performance of the optical scanning device. However, the optimization for all the necessary optical characteristics of the optical scanning device requires a large amount of time and effort, and it is difficult to determine the easy-to-manufacture, cost-effective design of the optical scanning device while the optical parameter optimization is obtained.
Suppose an optical scanning device that scans an image surface by focusing a deflected light beam from a rotary deflector which is rotated at a uniform-velocity. The scanning speed becomes relatively large (when compared to the uniform-velocity level) as the beam spot, created as a result of the focusing of the deflected light beam, approaches an outer peripheral end of the image surface in the main scanning direction. The linearity of the scanning light beam on the image surface has a positive value and it tends to be excessively large. A simple method for correcting the excessive linearity is that a meniscus lens “L” having a convex lens surface, facing a reflection point “P” of a rotary deflector, and a relatively small lens power is located adjacent to the rotary deflector, as shown in FIG. 6. When the meniscus lens “L” is used, the light beam from the rotary deflector is converted at the meniscus lens “L” into a refracted light beam as indicated by the dotted line in FIG. 6. It is possible to correct the excessive linearity at the outer peripheral end of the image surface so as to approach the uniform-velocity characteristics by using the meniscus lens “L”.
However, the convex and concave lens surfaces of the meniscus lens “L” are not concentric with respect to the reflection point “P” of the rotary deflector, and the use of the meniscus lens “L” is detrimental to increasing the optical system performance of the optical scanning device.